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Probing an SMPS signal which is referenced to a widely fluctuating voltage?

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treez

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Hi
Page 52 of the UCC28950 datasheet shows the COMP pin of the UCC28950 PSFB controller being probed with a scope probe. A 1k resistor is appended to the scope probe.
Can you please confirm that this 1k resistor is put there to prevent distortion of the COMP pin signal by the leakage current coming through the scope probe?
After all, the COMP pin is referenced to Diode bridge output negative, which swings down to -339V (below earth ground) 100 times per second.

UCC28950 datasheet
https://www.ti.com/lit/ds/symlink/ucc28950.pdf
 

To probe the circuit with a regular single ended oscilloscope input, the 400V DC power must be supplied by an isolated source, e.g. a lab power supply, or possible a rectifier circuit powered by a safety transformer.

The datasheet is assuming a test setup with grounded dc-, -339V is your imagination.

A different question is if you can make meaningful measurements with a good differential probe in the grid tied circuit.
 
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Not leakage per se but to give a degree of isolation of the probe capacitance from loading the feedback loop. It is only a recommendation as most x10 probes will already have some input resistor in series with the tip.

Brian.
 
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Thanks, sorry i forgot to say i was assuming an isolated AC source.

Thanks, i too suspected that they were worried about scope probe capacitance, though it surprises me, because the capacitances of the probe are usually very low...as in 8:30 of the following...

https://www.youtube.com/watch?v=OiAmER1OJh4
 
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Series resistors in a 100 to 200 ohm range are often placed at probe tips to reduce ringing with fast, low impedance signals. It's a kind of source side impedance matching.

The purpose of 1 k series resistor is to reduce high frequency noise injection by the probe, as already guessed. If we assume common mode currents running through the probe cable and ground clip, e.g. induced by power stage switching edges, even 10 or 20 pF probe input capacitance can have an effect and inject transients into the probed circuits. 1k series resistor provides some isolation for it.
 

The purpose of 1 k series resistor is to reduce high frequency noise injection by the probe

definitely - some people go up to 22k so as not to affect the ckt ... you get a filtered look - but still a look ...
 

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